1. Field of the Invention
The present invention relates to a distributed Doherty power amplifier, and more particularly, to a power amplifier, which exhibits broadband characteristics by connecting a first Doherty power amplifier and a second Doherty power amplifier in a distributed structure and exhibits a characteristic of offsetting tertiary distortion signals generated from the first Doherty power amplifier and the second Doherty power amplifier.
2. Description of the Related Art
Currently, a mobile communication system requires broad bandwidth and high linearity. Signals having broad bandwidth and high linearity exhibit a high peak-to-average power ratio (PAPR), and a power amplifier used in a base station or a relay base shall be operated at a large back-off power (BOP) to satisfy the high PAPR. However, a power amplifier operating in a large BOP domain exhibits a low efficiency, and, if an additional circuit is added for linearization of the power amplifier, the additional circuit further lowers efficiency of the overall mobile communication system.
Doherty power amplifiers exhibiting high efficiency at a relatively low output power are being researched to improve the efficiency of a power amplifier. In a Doherty power amplifier, a carrier amplifier and a peaking amplifier are connected to each other in parallel, and the efficiency of the Doherty power amplifier increases as load impedances of the carrier amplifier and the peaking amplifier vary according to magnitudes of input power. To change a load impedance of a Doherty power amplifier, a λ/4 transmission line is used in the output of the carrier amplifier. However, since a λ/4 transmission line is designed for one operation frequency, the λ/4 transmission line cannot function as an impedance changer with respect to another operation frequency, and thus it is difficult to maintain a high operation efficiency.
On the other hand, a distributed power amplifier is widely used to increase a bandwidth of a power amplifier. The distributed power amplifier not only exhibits a flat gain and a small reflection coefficient in a broad bandwidth, but also requires no power distributor and no power combiner for the input and output of two power amplifiers connected in parallel. However, the distributed power amplifier exhibits low output power and low efficiency.